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This commit expands the old Note [Type variable cycles in Givens] to apply
as well to Deriveds. See the Note for details and examples. This fixes a
regression introduced by my earlier commit that killed off the flattener in
favor of the rewriter.
A few other things happened along the way:
* unifyTest was renamed to touchabilityTest, because that's what it does.
* isInsolubleOccursCheck was folded into checkTypeEq, which does much of the
same work. To get this to work out, though, we need to keep more careful
track of what errors we spot in checkTypeEq, and so CheckTyEqResult has
become rather more glorious.
* A redundant Note or two was eliminated.
* Kill off occCheckForErrors; due to Note [Rewriting synonyms], the
extra occCheckExpand here is always redundant.
* Store blocked equalities separately from other inerts; less stuff
to look through when kicking out.
Close #19682.
test case: typecheck/should_compile/T19682{,b}
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This patch simplifies GHC to use simple subsumption.
Ticket #17775
Implements GHC proposal #287
https://github.com/ghc-proposals/ghc-proposals/blob/master/
proposals/0287-simplify-subsumption.rst
All the motivation is described there; I will not repeat it here.
The implementation payload:
* tcSubType and friends become noticably simpler, because it no
longer uses eta-expansion when checking subsumption.
* No deeplyInstantiate or deeplySkolemise
That in turn means that some tests fail, by design; they can all
be fixed by eta expansion. There is a list of such changes below.
Implementing the patch led me into a variety of sticky corners, so
the patch includes several othe changes, some quite significant:
* I made String wired-in, so that
"foo" :: String rather than
"foo" :: [Char]
This improves error messages, and fixes #15679
* The pattern match checker relies on knowing about in-scope equality
constraints, andd adds them to the desugarer's environment using
addTyCsDs. But the co_fn in a FunBind was missed, and for some reason
simple-subsumption ends up with dictionaries there. So I added a
call to addTyCsDs. This is really part of #18049.
* I moved the ic_telescope field out of Implication and into
ForAllSkol instead. This is a nice win; just expresses the code
much better.
* There was a bug in GHC.Tc.TyCl.Instance.tcDataFamInstHeader.
We called checkDataKindSig inside tc_kind_sig, /before/
solveEqualities and zonking. Obviously wrong, easily fixed.
* solveLocalEqualitiesX: there was a whole mess in here, around
failing fast enough. I discovered a bad latent bug where we
could successfully kind-check a type signature, and use it,
but have unsolved constraints that could fill in coercion
holes in that signature -- aargh.
It's all explained in Note [Failure in local type signatures]
in GHC.Tc.Solver. Much better now.
* I fixed a serious bug in anonymous type holes. IN
f :: Int -> (forall a. a -> _) -> Int
that "_" should be a unification variable at the /outer/
level; it cannot be instantiated to 'a'. This was plain
wrong. New fields mode_lvl and mode_holes in TcTyMode,
and auxiliary data type GHC.Tc.Gen.HsType.HoleMode.
This fixes #16292, but makes no progress towards the more
ambitious #16082
* I got sucked into an enormous refactoring of the reporting of
equality errors in GHC.Tc.Errors, especially in
mkEqErr1
mkTyVarEqErr
misMatchMsg
misMatchMsgOrCND
In particular, the very tricky mkExpectedActualMsg function
is gone.
It took me a full day. But the result is far easier to understand.
(Still not easy!) This led to various minor improvements in error
output, and an enormous number of test-case error wibbles.
One particular point: for occurs-check errors I now just say
Can't match 'a' against '[a]'
rather than using the intimidating language of "occurs check".
* Pretty-printing AbsBinds
Tests review
* Eta expansions
T11305: one eta expansion
T12082: one eta expansion (undefined)
T13585a: one eta expansion
T3102: one eta expansion
T3692: two eta expansions (tricky)
T2239: two eta expansions
T16473: one eta
determ004: two eta expansions (undefined)
annfail06: two eta (undefined)
T17923: four eta expansions (a strange program indeed!)
tcrun035: one eta expansion
* Ambiguity check at higher rank. Now that we have simple
subsumption, a type like
f :: (forall a. Eq a => Int) -> Int
is no longer ambiguous, because we could write
g :: (forall a. Eq a => Int) -> Int
g = f
and it'd typecheck just fine. But f's type is a bit
suspicious, and we might want to consider making the
ambiguity check do a check on each sub-term. Meanwhile,
these tests are accepted, whereas they were previously
rejected as ambiguous:
T7220a
T15438
T10503
T9222
* Some more interesting error message wibbles
T13381: Fine: one error (Int ~ Exp Int)
rather than two (Int ~ Exp Int, Exp Int ~ Int)
T9834: Small change in error (improvement)
T10619: Improved
T2414: Small change, due to order of unification, fine
T2534: A very simple case in which a change of unification order
means we get tow unsolved constraints instead of one
tc211: bizarre impredicative tests; just accept this for now
Updates Cabal and haddock submodules.
Metric Increase:
T12150
T12234
T5837
haddock.base
Metric Decrease:
haddock.compiler
haddock.Cabal
haddock.base
Merge note: This appears to break the
`UnliftedNewtypesDifficultUnification` test. It has been marked as
broken in the interest of merging.
(cherry picked from commit 66b7b195cb3dce93ed5078b80bf568efae904cc5)
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This puts the "Relevant bindings" section at the end.
It uses a TcErrors.Report Monoid to divide messages by importance and
then mappends them together. This is not the most efficient way since
there are various intermediate Reports and list appends, but it probably
doesn't matter since error messages shouldn't get that large, and are
usually prepended. In practice, everything is `important` except
`relevantBindings`, which is `supplementary`.
ErrMsg's errMsgShortDoc and errMsgExtraInfo were extracted into ErrDoc,
which has important, context, and suppelementary fields. Each of those
three sections is marked with a bullet character, '•' on unicode
terminals and '*' on ascii terminals. Since this breaks tons of tests,
I also modified testlib.normalise_errmsg to strip out '•'s.
--- Additional notes:
To avoid prepending * to an empty doc, I needed to filter empty docs.
This seemed less error-prone than trying to modify everyone who produces
SDoc to instead produce Maybe SDoc. So I added `Outputable.isEmpty`.
Unfortunately it needs a DynFlags, which is kind of bogus, but otherwise
I think I'd need another Empty case for SDoc, and then it couldn't be a
newtype any more.
ErrMsg's errMsgShortString is only used by the Show instance, which is
in turn only used by Show HscTypes.SourceError, which is in turn only
needed for the Exception instance. So it's probably possible to get rid
of errMsgShortString, but that would a be an unrelated cleanup.
Fixes #11014.
Test Plan: see above
Reviewers: austin, simonpj, thomie, bgamari
Reviewed By: thomie, bgamari
Subscribers: simonpj, nomeata, thomie
Differential Revision: https://phabricator.haskell.org/D1427
GHC Trac Issues: #11014
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This makes two real changes:
- Equalities like (a ~R [a]) really *are* insoluble. Previously,
GHC refused to give up when an occurs check bit on a representational
equality. But for datatypes, it really should bail.
- Now, GHC will sometimes report an occurs check error (in cases above)
for representational equalities. Previously, it never did.
This "fixes" #10715, where by "fix", I mean clarifies the error message.
It's unclear how to do more to fix that ticket.
Test cases: typecheck/should_fail/T10715{,b}
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